Selenium rectifier



Aug- 7, 1956 w. F. BONNER ETAL 2,758,266

SELENIUM RECTIF'IER Filed Aug. 9, 1954 ALCOHO a PAR 7'5 /Vy ON 506C NYLON conce/WHA ns 300 O CC CARR/ER Se o2 45 cc Acqua? AcEr/c 3.75 Gle/Ms Acva INVENTORS /vr/fo/v y E MiN/QQ A BY @545% ATTORNEY SELENIUM RECTIFIER Walter F. Bonner, Glen Ridge, Robert F. Durst, Orange, William Lewanda, Fairlawn, and Anthony E. Machala, Bayonne, N. I., assignors to International Telephone and Telegraph Corporation, New York, N. Y., a corporation of Maryland Application August 9, 1954, Serial No. 448,562

l1 Claims. (Cl. 317-241) This invention relates to improvements in selenium rectifiers and more particularly to the barrier layer introduced between the selenium and counterelectrode layers thereof.

In U. S. Patent No. 2,660,698, of D. W. Black issued November 24, 1953 for Selenium Rectifier there is described a barrier layer consisting essentially of a polyamide resin, which for example may be one of the class of compounds commonly known by the trademark name Nylon. The preferred form of Nylon from which the carrier layer is made is one soluble in the lower aliphatic alcohols and may be defined as a high molecular weight linear polymeric carbonamide soluble in the lower aliphatic alcohols. One such soluble Nylon is designated as FM-650l by the Du Pont Corporation.

After a selenium cell has been constructed, a current is passed through the cell in the reverse direction, that is the direction of greater resistivity and as this current flows the resistance of the cell in the reverse direction increases until it reaches a desired value. Depending upon the nature of the materials employed, a cell can be built up to different resistance values. This process of sending a current through a cell in the reverse direction to build up its resistance is called forming al cell and usually takes many hours.

In order to improve the forming time it has been suggested in the copending application of the Serial No. 264,274 filed December 29, 1951, now Patent No. 2,724,078, by Robert F. Durst et al. for Selenium Rectifiers that selenium dioxide be added to the Nylon solution employed. Since the prime limiting factor in the mass production of selenium rectifiers has been that of forming time, a great deal of effort has been expended in attempting to lessen the forming time as well as to have more uniform forming of individual rectiiiers.

An object of the present invention is the provision of an improved selenium rectifier.

Another object of the present invention is the provision of such a rectifier which enables improving the forming operation.

In accordance with the main feature of the present invention, it has been discovered that the addition of acetic acid (a low molecular weight aliphatic acid), within certain critical limits, to the solution from which the barrier layer is deposited reduces the forming time to. a relatively short period. Furthermore greater uniformity in the electrical properties of the rectifiers thus produced is obtained.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood, reference being had to the drawings in which:

Fig. 1 is a cross sectional view of a rectifier showing one embodiment of the invention; and

Fig. 2 is a chart of the ingredients used in forming the barrier layer lacquer.

Referring now to Fig. l, the selenium rectifier there tes "arent illustrated comprises a base plate 1, a selenium layer 2 having an artificial or non-generic barrier layer 3 formed thereon, and on top of the barrier layer a counterelectrode 4. While the other elements of the rectifier cell may be conventional and conventionally processed, the present invention is directed to the artificial barrier layer 3 and to the lacquerfrom. which it is formed.

As is more fully described in. said U. S. Patent No. 2,660,698, the lacquer there used for producing the artiiicial barrier layer comprises broadly a soluble Nylon to which is added, as a carrier, a nitroparaflin. The resulting solution` is then treated by adding thereto ammonium hydroxide in an amount sufiicient to raise the normal pH of the composition to a value of approximately 7. In the above-mentioned copending application of Durst et al., Serial No. 264,274, it is proposed to add selenium dioxide to the resulting lacquer in the amounts of from .01% to 1% by weight of the total lacquer. In accordance with the present invention, we have found that the forming time is remarkably reduced when acetic acid is added to the lacquer in the amounts of from 0.5 to 10% by volume of total lacquer. It has been furthermore found that with these values of acetic acid the selenium dioxide may be raised to as much as 3.0% by weight of total lacquer.

A preferred example of the formation ofthe barrier layer according tothe present invention is asfollows, reference being had to the chart of Fig. 2. A Nylon concentrate is made up of the following:

30 parts by wgt, isopropyl alcohol 6' parts by wgt. of water 4 parts by wgt. of furfuryl alcohol 2 parts by wgt. Nylon FM-6501 The above concentrate is weighed into a flask and refiuxed gently until a clear solution is obtained.

A carrier for the concentrate consisting of a 1 to 3 mixture by volume of nitromethane and isopropyl alcohol is prepared by adding a liter of nitromethane to three liters of isopropyl alcohol.

Fifty cubic centimeters of Nylon concentrate is added to 3,000 cc of the isopropyl `alcohol-nitromethane mixturet and then mixed thoroughly for example by stirringv with a glass rod.

The pH of the solution is adjusted to 7 by the dropwise addition of a solution comprising a mixture of 1 part by volume of a concentrated solution (28% NH3) of ammonia to` 2 parts by volume of distilled Water. 3.75 grams, 1.25% by Weight of' total lacquer, of sele nium dioxide is added to the solution and stirred until completely dissolved. Note that the limits of selenium dioxide may be varied from 0.1 to 3.0% by Weight of total lacquer.

Forty-.five cubic centimeters of concentrated acetic acid (1.5% by volume of total lacquer)l is added and mixed thoroughly. Note that the acetic acid may be varied from 0.5 to 1,0%. The resultantsolution may be filtered and kept until ready for use inthe manufacture of the selenium cell.

In the above solution the amount of acetic acid is about 25 timesl the weight of Nylon. The proportion of Nylon to the sum of all other ingredients is about 0.06%.

Furfuryl alcohol is used in addition to the isopropyl because it affords a more rapid solution of the Nylon, but it could be omitted. Likewise the isopropyl alcohol could be replaced by methane or ethyl alcohol or any 10W chain aliphatic alcohol.

In the making of the rectifier cell, the selenium is applied to a base plate in a conventional manner and treated to produce the proper crystalline form, the latter treatment usually consisting of heat and pressure.

The lacquer herein described is then applied in any suitable way such as for example by spraying. The lacquer is permitted to dry, preferably at room temperature, and thereafter the usual counterelcctrode is applied in a conventional manner. The barrier layer remaining on the selenium is a reaction product of some or all of the components also involving the adjacent rectifier layers.

'Ihe cell is formed by passing current therethrough in the high resistance direction usually as a continuous or pulsating direct current, until the cell has reached the desired high resistance.

With the process herein described, cells have been formed uniformly to a voltage of between 60 and 65 Volts D. C. While the forming current may be varied, it is preferred to apply a current of pulsating D. C., as known in the art. During the forming, the cells may be immersed in oil for cooling purposes and the oil bath is maintained between the temperatures of 40-70 C. approximately.

While heretofore in the manufacture of such type cells the forming time has varied from to 24 hours with the addition of the acetic acid, the time is reduced to from 2 to 3 hours.

While the exact function of acetic acid in shortening the forming time of rectiiiers may not be fully understood, and this information is not essential to the present invention, nevertheless one theory might be propounded explaining the action thereof. Acetic acid having a low ionization constant tends to act as a buffer to maintain the pH of the lacquer within a narrow range regardless of any other chemical changes which slowly take place in the lacquer layer. Ammonium acetate formed by the interaction of acetic acid and the ammonium hydroxide in the solution is a standard buiering agent. Even the polyamide itself (the Nylon) can react with the acetic acid to form bulering salts. 'Ihis stabilization of pH due to buffering action helps to stabilize the forming behavior of the rectiiers.

With regard to the above described lacquer, it is not essential that nitromethane be used and in place thereof other suitable nitroparans such as nitroethane or nitropropane may be employed.

With regard to the use of selenium dioxide, while it is not essential for a rectier of this type, it does help in improving the forming time and is preferred for that reason. The characteristics of the rectifier are a function of the pH and the forming voltage, therefore the pH and forming voltage should be adjusted for various types of rectiers in accordance with the requirements and specifications of the job for which it will be used, and the pH need not be 7 for all cases.

Furthermore while one form of soluble Nylon has been described as used in the lacquer, other forms of Nylon soluble in the lower aliphatic alcohols may be employed. The limitation as to the type of Nylon employed is that it should be soluble in rapid drying solvents.

While we have described the use of isopropyl and furfuryl alcohol as part of the carrier in which the Nylon is mixed, other solutions may be employed as the carrier such as for example, 100% ethyl or methyl alcohol, or any other lower aliphatic alcohol and the water can be omitted.

The proportions specified for the hereinabove described concentrate and mixture may be varied at least within the limits set forth in said U. S. Patent No. 2,660,698.

While we have described above the principles of our invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of our invention as set forth in the objects thereof and in the accompanying claims. n

What is claimed is:

l. In a selenium rectifier having a base plate, a layer of selenium on the base plate, a counterelectrode layer on the selenium, a barrier layer between the selenium and the counterelectrode, said barrier layer being deposited from a solution of a high molecular weight linear polymeric carbonamide soluble in the lower aliphatic alcohols, characterized in that said solution includes acetic acid in the amount of 0.5 to 10% by volume of total lacquer.

2. A selenium rectier having a base plate, a layer of selenium on the base plate, a counterelectrode layer on the selenium and a barrier layer between the selenium and the counterelectrode deposited from a solution consisting essentially of a high molecular weight linear polymeric carbonamide soluble in the lower aliphatic alcohols, a carrier for said carbonamide and acetic acid contained in the solution in the amount of 0.5 to 10% by volume.

3. A rectiier according to claim 2 in which the solution has been adjusted to a pH of 7.

4. A rectifier according to claim 2 in which the solution contains selenium dioxide from .01 to 3% by weight of total lacquer.

5. A rectifier according to claim 2 in which the carrier includes a nitroparaiiin.

6. A rectifier according to claim 2 in which the solution includes a nitroparaflin and at least one of the lower alcohols.

7. A rectifier having a base plate, a layer of selenium on the base plate, a counterelectrode layer on the selenium and a barrier layer between the selenium and the counterelectrode deposited from a lacquer containing between 0.5 to 10% of acetic acid by volume of total lacquer.

8. In a process of making a selenium rectiiier, the steps of producing a barrier layer between the selenium and the counterelectrode comprising dissolving a high molecular weight linear polymeric carbonamide of the type soluble in the lower aliphatic alcohols in a solution containing at least one of the lower alcohols, adding to said solution 0.5 to 10% acetic acid and depositing the lacquer thus produced on the selenium, and evaporating the solution before applying the counterelectrode.

9. In a selenium rectifier having a base plate, a laye: of selenium on the base plate, a counterelectrode layer on the selenium, a barrier layer between the selenium and the counterelectrode, said barrier layer being deposited from a solution of a high molecular Weight linear polymeric carbonamide soluble in the lower aliphatic alcohols, and acetic acid in said solution approximately 25 times the weight of the carbonamide.

10. A rectifier according to claim 9 wherein the proportion of the carbonamide to the sum of all other ingredients is approximately 0.06%.

l1. In a selenium rectitier having a base plate, a layer of selenium on the base plate, a counterelectrode layer on the selenium, a barrier layer between the selenium and the counterelectrode, said barrier layer comprising principally the reaction product of acetic acid, a high molecular weight linear polymeric carbonamide soluble in the lower aliphatic alcohols, and ammonia, which is approximately 60% of the weight of the barrier layer, the remainder thereof consisting essentially of selenium dioxide and cadmium-containing counterelectrode alloy. 

1. IN A SELENIUM RECTIFIER HAVING A BASE PLATE, A LAYER OF SELENIUM ON THE BASE PLATE, A COUNTERELECTRODE LAYER ON THE SELENIUM, A BARRIER LAYER BETWEEN THE SELENIUM AND THE COUNTERELECTRODE, SAID BARRIER LAYER BEING DEPOSITED FROM A SOLUTION OF A HIGH MOLECULAR WEIGHT LINEAR 